Cisco ASR 9000 Series Configuration Manuallines page 244

Overview
• RPL neighbor node—The RPL neighbor node is an Ethernet ring node adjacent to the RPL. It is responsible
for blocking its end of the RPL under normal conditions. This node type is optional and prevents RPL
usage when protected.
• RPL next-neighbor node—The RPL next-neighbor node is an Ethernet ring node adjacent to RPL owner
node or RPL neighbor node. It is mainly used for FDB flush optimization on the ring. This node is also
optional.
The following figure illustrates the G.8032 Ethernet ring.
Figure 25: G.8032 Ethernet Ring
Nodes on the ring use control messages called RAPS to coordinate the activities of switching on or off the
RPL link. Any failure along the ring triggers a RAPS signal fail (RAPS SF) message along both directions,
from the nodes adjacent to the failed link, after the nodes have blocked the port facing the failed link. On
obtaining this message, the RPL owner unblocks the RPL port.
Note A single link failure in the ring ensures a loop-free topology.
Line status and Connectivity Fault Management protocols are used to detect ring link and node failure. During
the recovery phase, when the failed link is restored, the nodes adjacent to the restored link send RAPS no
request (RAPS NR) messages. On obtaining this message, the RPL owner blocks the RPL port and sends
RAPS no request, root blocked (RAPS NR, RB) messages. This causes all other nodes, other than the RPL
owner in the ring, to unblock all blocked ports. The ERP protocol is robust enough to work for both
unidirectional failure and multiple link failure scenarios in a ring topology.
A G.8032 ring supports these basic operator administrative commands:
• Force switch (FS)—Allows operator to forcefully block a particular ring-port.
• Effective even if there is an existing SF condition
• Multiple FS commands for ring supported
• May be used to allow immediate maintenance operations
• Manual switch (MS)—Allows operator to manually block a particular ring-port.
• Ineffective in an existing FS or SF condition
• Overridden by new FS or SF conditions
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
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Implementing Multipoint Layer 2 Services